Don't ignore all exceptions during coercion for nullable fields

Only ignore exceptions if a value for a nullable field is not null.
Previously all exceptions during custom coerction were ignored with nullable
fields. The developer and/or user most likely wants to know about
exceptions that happen during coercion - this leads to hard to debug
problems otherwise.

The purpose of the `e is not TypeError` condition was unclear. As far as I
know this condition can never be false (`e` is an instance of an exception,
not an exception class).

I think this is still in line with the original intent of this code, which
was to make common coercions like `float` work with nullable fields out
of the box: #269

cerberus/tests/test_normalization.py

@@ -170,6 +170,15 @@ def test_nullables_dont_fail_coerce():
     assert_normalized(document, document, schema)
 
 
+def test_nullables_fail_coerce_on_non_null_values(validator):
+    schema = {'foo': {'coerce': lambda x: 1 / 0, 'nullable': True, 'type': 'integer'}}
+    document = {'foo': None}
+    assert_normalized(document, document, schema)
+
+    validator({'foo': 2}, schema)
+    assert errors.COERCION_FAILED in validator._errors
+
+
 def test_normalized():
     schema = {'amount': {'coerce': int}}
     document = {'amount': '2'}

cerberus/validator.py

@@ -723,7 +723,7 @@ def __normalize_coerce(self, processor, field, value, nullable, error):
         try:
             return processor(value)
         except Exception as e:
-            if not nullable and e is not TypeError:
+            if not nullable or value is not None:
                 self._error(field, error, str(e))
             return value